The private pilot said she was taxing her single-engine airplane to the active runway when the engine suddenly went to full power. She did not recall any engine instrument indications and tried to reduce power by closing the throttle, pulling the mixture control, and applying the brakes. During this time, the airplane veered off the taxiway, impacted a snow bank, and flipped over on its back causing substantial damage to the airframe. Examination of the engine found no mechanical anomalies that would explain why there was a sudden increase in engine power. The pilot had 2,000 feet of taxiway remaining from where the airplane departed the runway.